CONDUCT of CARDIOVASCULAR PERFUSION; An OVERVIEWPerfEd International Reprinted: Vinas, M.S., Chapter 3 "Extracorporeal Circulation," Kambam, J., Editor, Cardiac Anesthesia for Infants and Children, the C.V. Mosby, Co., St. Louis, MO, 1994 The application of extra-corporeal circulation (ECC) was pioneered by the experiments of Dr. Jack Gibbon in 1934 at Massachusetts General Hospital. However, it was not until 1953 that the first successful human open-heart procedure, closure of an atrial septal defect, was performed by this surgeon on a young female patient (Bone). It is estimated that there are 750,000 cardiopulmonary bypass (CPB) procedures performed annually in the United States. Of this number approximately 6% involve infant/ pediatric cardiac surgical intervention (AHA). A 1990 Pediatric Perfusion survey cited a 127 responses from community, government and university medical centers performing infant/ pediatric open-heart surgery. The centers reported performing 14,473 open-heat surgery (OHS) procedures requiring extra-corporeal circulation (Hill). Stammers and Riley compiled data on over 1016 infant/ pediatric treated with cardiopulmonary bypass (CPB) at University of Michigan Hospitals, between 1986-1990, 869 were able to be placed into sixteen categories. The majority of the surgical procedures consisted of the following: INFANT/ PEDIATRIC SURGICAL PROCEDURES/ PERCENTILE 9.3 %Atrial Septal Defect

2.1 %Atrial/ Septal Ventricular Defect

8.2 %Atrioventricular Canal

5.2 %Conduction Disturbance

3.1 %Double Outlet of the Right Ventricle

9.0 %Left Ventricular Outflow Tract

2.1 %Heart Transplants

6.6 %Hypoplastic Left Heart Syndrome

2.9 %Mitral Valve Lesions

1.8 %Partial Anomalous Pulmonary Venous Return

3.5 %Pulmonary Atresia

7.9 %Pulmonary Stenosis

12.9 %Tetrology of Fallot

12.5 %Transposition of the Great Arteries

2.3 %Truncus Arteriosus

10.7 %Ventricular Septal Defect

The application of extracorporeal circulation requires careful assessment of hematologic values and the effects of hemodilution and hypothermia. All attempts should be made to design an accommodating extracorporeal circuit while minimizing, preferrably eliminating, blood product requirements.

HEMOGLOBIN/ HEMATOCRIT The hemoglobin in infant and pediatric patient's has been surveyed and reported between the ranges of 12.5-22 gm./dL (Hartley-Winkler). Hemodilutional calculations, however, are predicated on hematocrit values. AGE HEMOGLOBIN VALUES 1 Day 18-22 GM/DL 2 Weeks 17 GM/DL 3 Months 10 GM/DL 3-5 Years 12.5-13 GM/ DL A non-haemic or asanguinous prime of the heart-lung console circuit may dilute an infant with a standard hematocrit to a calculated value less than 15%, a polycythemic individual not as critical. It is the cardiovascular perfusionist's responsibility to estimate, via calculations, the degree of hemodilution and any blood products requirements needed to adjust the hematocrit to a value of 20-30% or greater during ECC. Non-haemic primes with circulating hematocrit's of 13-18% have been reported (Berryessa, Conley, Hartley- Winkler, McCormick, Stammers). Our hemodilutional protocol requires a 30% packed cell volume (PCV) during ECC for infants, a 25% PCV for pediatric patients. To augment the hematocrit during ECC, an online ultrafilter may be integrated into the circuit (Han, Molina, Tamari). Ultrafiltration, in the infant/ pediatric circuit, promotes the elimination of excessive extra-cellular solutions less than 17,000 Daltons at a maximum rate of 10-30 ml./min. (Minntech). The hematocrit at the termination of ECC should average 25-30% or greater to enhance available oxygen carrying capacity.

PLASMA VOLUME Plasma normally comprises 55-65% of the estimated blood volume. A 3.0 kilogram infant, for example, may have a plasma volume of 180 ml. A 700 ml. non-plasma prime would dilute the respective clotting factors, fibrinogen and platelets below 20%, which is inadequate for proper hemostasis. The plasma volume in infants/ pediatrics should be calculated to assess hemodilution of these factors, with the possibility of adding fresh frozen plasma (FFP) to the extra-corporeal circuit.

HEMODILUTION/ TARGET HEMATOCRIT Infant and Pediatric patients have a higher blood volume factor than the adult, this value decreases with human development. Institutionally we have adopted the following blood volume factors: PATIENT AGE BLOOD VOLUME ml./ Kg.

0-3 months 90

3-6 months 85

10-2 months 80

> 12 months 70

Several calculations are required to assess hemodilution and blood product requirements. The following depicts a patient with a weight of 10 kilograms, blood volume factor of 85 ml./Kg., hematocrit of 40% (HCT1) and an ECC priming volume of 700 ml. The resultant calculations will estimate the patient's blood volume (PBV), patient red cell volume or mass (RCM1), total system or ECC circulating volume (TSV), hemodilutional hematocrit (HCT2) and hemodilutional or ECC circulating red cell mass (RCM2) required (Vinas): HEMODILUTIONAL CALCULATIONS PBV = 850 ml. = 85 ml./Kg. x 10 Kilograms

RCM1= 340 ml. = 850 ml. (PBV) x .40 (HCT1)

TSV = 1550 ml. = 850 ml. (PBV) + Prime Volume

HCT2 = 22 % = 340 ml. (RCM1) / 1550 ml. x 100

RCM2 = 465 ml. = 1550 ml. (TSV) x 0.30

RCM1-2 = -125 ml. = 340 ml. (RCM1) - 465 ml. (RCM2)

125 ml. RCM is required to be added to the ECC prime to achieve a circulating hematocrit of 30%. The relative body size, compared to the perfusate, required to prime the extra- corporeal circuit, will cause a neonate or infant to be more adversely affected by hemodilution than a pediatric. INFANT PEDIATRIC ADULT

Body Weight (Kg.) 5 25 70

Blood Volume Factor (ml./Kg.) 85 70 70

Estimated Blood Volume (ml.) 425 1750 4900

Hematocrit (%) 40 40 40

Estimated Red Cell Mass (ml.) 170 700 1960

Estimated Plasma Volume (ml.) 255 1050 2940

Prime-Extracorporeal circuit (ml.) 700 1000 1750

Calculated Hemodilutional Hct. (%) 15 25 30

FIBRINOGEN A critical consideration is plasma fibrinogen dilution. Normal plasma fibrinogen levels are 150-400 mg./dL (Ecklund). The infant/ pediatric patient's relative low blood volume with priming requirements of the ECC circuit causes the fibrinogen concentration to be adversely diluted. During CPB, it is desirable to maintain the plasma fibrinogen concentration above 100 mg./dL. in order to prevent impairment of post-ECC hemostasis (Ecklund, Pfefferkorn p. 61, Taylor p.274). Given an example of a 10 Kilogram patient with a patient blood volume of 850 ml., a pre-bypass hematocrit of 40%, fibrinogen level of 250 mg./dL. and a 700 ml. ECC prime volume, additional fibrinogen via FFP is needed. Given the previous parameters mentioned, the calculation of fibrinogen dilution, and subsequent reconstituting, would be calculated as follows: Plasma Volume= 5.1 dL.= [850 ml. x [1.0-(Hct/100)]] x .01

Total Plasma Fibrinogen= 1275 mg.= 250 mg./dL Pt. Fibrinogen level x 5.10 dL

Total System Volume= 15.5 dL= (1550 ml./1000) x 10

Total System Fibrinogen= 82 mg./dL= 1275 mg./ 15.5 dL

The calculated fibrinogen value derives a deficit of 18 mg./dL. x 15.5 = 279 mg. If Fresh Frozen Plasma, diluted with CPD-A contains 200 mg./ 100 ml. (dL) of Fibrinogen, an additional 140 ml. of fresh frozen plasma should be administered to raise the circulating level to 100 mg./dL (Pfefferkorn pp. 61-62). PLATELETS Platelet concentration values range from 150,000-400,000/ cu. mm. (Reed and Stafford p.127). The classification of thrombocytopenia is given to levels less than 100,000/ cu. mm.. Levels below this value may result in prolonged bleeding times (Orland p. 273). Blood exposure to the foreign components of the extracorporeal circuit may deplete platelet concentrations. This phenomenon may be observed in electron photomicrographs of the arterial filter and blood reservoirs. Albumin, added to the ECC prime, reduces platelet aggregation on these foreign surfaces thus minimizing loss to the circuit components (Gurjar, Hedlund). Platelet reconstitution, if indicated, should be reserved until post-bypass in order to optimize the effects of the platelets and their effect on hemostasis. CRYSTALLOID SOLUTIONS In the event that crystalloid priming solutions are administered they should be pH balanced and isotonic. The electrolyte composition should approach the values of normal blood chemistries to ensure normal electrolyte levels. Calcium Chloride may be added to solutions that do not contain the electrolyte to a value of 10 mg./ dL or 100 mg./L. Blood chemistries such as Sodium, Potassium, Magnesium, Chloride, Glucose, Total Protein, Total and Ionized Calcium along with hematologic values should be monitored every 20-30 minutes during cardiopulmonary bypass. Patient's with relatively high potassium levels, renal failure for example, should not be administered potassium containing solutions, at least caution is advised. 0.9% Sodium Chloride solutions, buffered to a pH of 7.4, may be substituted. Lactated Ringers solution should not be administered to patients exhibiting clinical signs of lactic acidosis.

ALBUMIN/ PLASMA ONCOTIC PRESSURE Of the plasma proteins albumin exerts the main influence on oncotic pressure which is 25 mm. Hg. The average albumin molecule has a molecular weight of 69,000 and is half the size of the average globulin molecule (Berne p.147). The colloid osmotic or oncotic properties of protein balances hydrostatic and oncotic pressure gradients across the capillary walls. Normal COP is approximately 20-25 mm. Hg. Values below 17 mm. Hg. may lead to pulmonary edema. COP levels approaching 15 mm. Hg. have been observed without incidence during CPB (Beshere). "Third spacing", the migration of intravascular fluid into the interstitum causing edema should be realized as a possibility during ECC, especially in the infant and pediatric populations. This phenomenon may be deterred by the use of an isoncotic (protein) priming solution. Reductions of the COP by 30-60% during ECC have been reported without chronic complications in the adult patient (Beshere). Pediatric patients, not requiring packed red blood cells, or fibrinogen via FFP may be administered an isotonic physiologic solution such as Plasmalyte-A, Normosol, or Lactated Ringer's, however, the protein dilution should be considered (McCormick). Each ml. of 12.5 gms., 25% Albumin is oncotically equivalent to five times its volume of normal human plasma (American Red Cross). 50 ml. of 25% Albumin may be included to reconstitute each 250 ml. of crystalloid perfusate to an isoncotic solution. Total serum protein levels range between 6.0-7.8 gm./dL, Albumin levels 3.2-4.5 gm./dL (Tilkian). Acceptable total protein levels of 4.0 gm./dL have been reported during cardio-pulmonary bypass. Total serum protein levels should be carefully monitored during I.V. albumin administration to ensure isoncotic levels. The following formula may be used is for correction of an oncotic deficit if the total serum protein (TSP) level, in gm. dL, has been measured and the plasma volume calculated in deciliters (American Red Cross). (Desired-Actual TSP in gm./dL) x Plasma Volume (dL) x 2 HESPAN 6% Hetastarch, is synthesized by the hydroxethylation of polysaccharides and approximates the behavior of Albumin. It has been used successfully for non-protein primes of the ECC circuit and as an agent to increase the colloid osmotic pressure in patient's exhibiting hypovolemia circulatory shock (Haupt, Palanzo). However, being a non-protein agent; plasma protein, Albumin and/or COP levels should be monitored along with ionized calcium when employing this agent. CARDIOPULMONARY PRIMING COMPOSITIONS The compositions of priming volumes are as varied as the institutions performing OHS procedures. In addition to, or in lieu of, banked blood products some institutions may use Lactated Ringer's, Normosol-R or Plasmalyte-A. To these they may add Mannitol or Lasix, 5% Dextrose, 6% Hetastarch, Albumin, Sodium Bicarbonate, Sodium Heparin, the compositions are infinite (Chui, Hartley-Winkler, Molina, Palanzo, Reed and Stafford p.260, Stammers, Taylor pp.229- 236).

BODY SURFACE AREA (B.S.A.) The DuBois and DuBois Body Surface Area chart is used to establish Body Surface Area predicated on height and weight. The chart is not sex specific. The infant/ pediatric version, as well as the adult, is based one of the following equations: BSA M2 = [(Kg.) ^ 0.425 x (Cm.) ^ 0.725] x 0.007184 or BSA M2 = SQR. RT. [(Kg. x Cm.)/ 3500] The BSA in square meters is essential in calculating extra-corporeal circulation flowrates. AGE (yr.) WEIGHTKg./ Lbs. HEIGHTCm./In. BSAM2 Kg./M2

Infants 0.0-0.50.5-1.0 6/ 139/ 20 60/ 2471/ 28 0.320.42 18.7521.43

Children 1- 34- 67-10 13/ 2920/ 4428/ 62 90/ 35112/ 44132/ 52 0.570.791.01 22.8025.3027.70

Adult 18-45 70/ 154 178/ 70 1.89 37.00

Adopted from The Merck Manual, 15th Edition, 79:900, 1987.

BASAL OXYGEN CONSUMPTION The basal oxygen consumption (ml./min.) in the infant be 5-7 times the value of an adult, however, the MET levels (ml./min./Kg.) is about twice that of the adult (Galletti). The Respiratory Quotient (VCO2/VO2) may be reduced by 15-20% due to anesthesia, skeletal muscle paralysis and mechanical ventilation Hypothermia during ECC further reduces this value (Riley and Justison, Mitchell). BASAL OXYGEN CONSUMPTION (VO2) vs. BODY WEIGHT VO2ml/m/Kg.Averageml/m/Kg.Kg.METSl./min.AverageVO2

7.5-9.585.0552.4342.5

7.5-9.008.25102.3682.5

6.5-8.507.50152.00112.5

6.0-7.56.75201.93135.0

5.5-6.56.00251.71150.0

5.0-6.05.50301.57165.0

4.5-5.55.00351.43175.0

4.5-5.04.75401.36190.0

ADULT 4.0-5.03.50701.00250.0

Adopted from: Galletti, P.M. and Brecher, G.A., Heart- Lung Bypass: Principles and Techniques of Extracorporeal Circulation, Grune & Stratton, New York; 1962.

Oxygen consumption is normally derived from the Fick equation. This method calculates the arterial and venous oxygen content difference and multiplies that value by the cardiac output in L/M x 10 (Bolen, Miller). VO2 (ml./min.) = (CaO2-CvO2) x CO x 10 During ECC, if the Hgb, C.O. and A/V venous saturations are known, oxygen consumption may be calculated without knowing the PO2 values since dissolved oxygen normally contributes less than 0.3 Volumes %. of the arterial O2 content. VO2 (ml./min.) = Hb. x 1.34 x [(SaO2-SvO2)/100] x CO x 10 The basal oxygen consumption of a neonate may vary due to a variety of factors. Extracorporeal flowrate requirements are predicated on the predicted basal and hypothermic oxygen requirements, level of anesthesia, degree of hemodilution, oxygen carrying capacity, degree of hypothermia etc.

EXTRACORPOREAL PERFUSION FLOWRATES Pediatric perfusion groups have reported the application of extracorporeal perfusion flowrate ranges between 1.80-3.5 L/Min./M2, others use 70-150 ml/Kg/Min. (Berryessa, Chui, Hartley-Winkler, Mitchell, Molina, Page p.6-1, Pfefferkorn p. 64, Reed and Kuruz p.140, Reed and Stafford p.406). Flowrate guidelines have been adopted from the American Association for Extra-Corporeal Technology's (AmSECT) publication, Pediatric Perfusion, Paul A. Page, PA, CCP. The higher of the two calculations is considered the optimal and the lower value is considered the minimum flowrate. ECC FLOWRATE - L/M2 ECC FLOWRATE ml./ Kg.

Newborns to2 yrs. = 2.6 x BSA2- 5 Kg. = 150 ml./Kg.

2 - 4 yrs. = 2.5 x BSA6-10 Kg. = 125 ml./Kg.

4 - 6 yrs. = 2.4 x BSA11-15 Kg. = 100 ml./Kg.

6 - 9 yrs. = 2.3 x BSA16-25 Kg. = 90 ml./Kg.

> 9 yrs. = 2.2 x BSA26-35 Kg. = 80 ml./Kg.

> 35 Kg. = 70 ml./Kg.

Extracorporeal perfusion flowrates are adjusted during hypothermia to ensure adequate arterial/ venous oxygen transferability. Oxygen consumption in humans decreases normally at a rate of 7% per degree Celsius . Therefore, decreasing the temperature from 37 C to 30 C would reduce oxygen consumption requirements to 50%, 25% at 23 C. A pediatric patient with a calculated basal VO2 of 84 ml./min. at 37C may experience a reduction to 42 ml./min. at 30 C, 21 ml./min. at 23 C via hemodilution and hypothermia (Reed and Stafford p. 325). Other investigators have reported a 50-60% reduction in total body oxygen consumption at 28 C to 30 C a 80-90% reduction at 18 C to 20 C and 90% between 8 to 10 C (Greeley, Mitchell). Formulas are subject to estimations and do not consider alterations in cardiopulmonary and hemodynamic pathophysiology. Adequacy of extracorporeal perfusion flowrates and accommodation of oxygen metabolic requirements mandates frequent, if not continuous, arterial and venous blood gas analysis in conjunction with hematology and chemistry values. The assessment of oxygen delivery vs. oxygen consumption is advisable to determine the adequacy of perfusion flowrates and/or assessment of anesthesia levels.

PULSATILE vs. NON-PULSATILE PERFUSION Pulsatile vs. non-pulsatile perfusion has been debated for several years. The "pulse wave" is generated by a specially designed dual roller pump which may be programed to deliver an intermittent flow that emulates an arterial pressure waveform. Proponents of pulsatile perfusion cite improved perfusion to the vital organs, better distribution to the tissues during warming and cooling, enhanced oxygen utilization by the vital organs, decrease in lactic acid, and systemic vascular resistance, among others (Berryessa, Casper, McCormick, Taylor p.76). Some researchers dispute the value of pulsatile flow and classify it's application as being detrimental (Taylor p.77). It is recommended that pulsatile perfusion should not be performed with a microporous membrane oxygenator that is placed distal to the arterial pump. Gas may be transported across the micropores of the membrane during the negative phase of the pulse wave (Reed and Kuruz p.109). CEREBRAL FUNCTION Cerbral function monitors are utilized to monitor the amplitude and frequency of cerebral electrical activity during ECC. Cerebral activity is dependent on the level of anesthesia, arterial pressure as well as the degree of hypothermia. Cerbral electrical activity (EEG waves) is minimized when the arterial pressure during ECC is below 35 mm. Hg. and during circulatory arrest at 18 C core temperature (Greeley, Taylor p. 25). Cerebral function monitoring should be considered as an adjunct to anesthetic, ECC and hypothermic management (Johnson, Kern, Murkin). Recently, investigators have been able to measure cerebral blood flow (CBF) and cerebral metabolic rate for oxygen (CMRO2) in the neonate, infant and pediatric patients. This has been by direct measurement before, during, and following hypothermic cardiopulmonary bypass, with and without deep hypothermic arrest (DHCA). Brain ischemic tolerance during hypothermia, referred to as hypothermic metabolic index (HMI), has been developed by Greeley et. al. Based on the HMI they have reported a predictable "safe" cerebral ischemic time of 11-19 minutes at 28 C and 39-65 minutes at 18 C. EXTRACORPOREAL CIRCUITRY The components selected for the extracorporeal circuit tubing and components are predicated on minimizing hemodilution while being able to accommodate blood flowrate requirements without excessive resistance to perfusion flowrates. The tubing comprising the extracorporeal circuit is manufactured of clear polyvinyl chloride. The cannula, catheter and tubing connectors, as well as the casings of the oxygenators, arterial filters and hemoconcentrators usually consist of clear or opaque Polycarbonate. The ECC circuit is usually comprised of the arterial line, arterial pump boot, venous line, suction lines, a bubbler or membrane oxygenator. A cardiotomy reservoir is required with certain oxygenators (Malinaukas). Arterial filters should be incorporated to filter particulate matter between 20-40 microns as well as gaseous microemboli during ECC (Butler, Demierre, Hill, Massimino, Taylor p.369). The cardiotomy allows for the recycling and filtration of blood suctioned from the surgical field.

DUAL ROLLER PUMPHEAD vs. CONSTRAINED VORTEX PUMP The device which generates forward flow in the extracorporeal circuit is the perfusion pump which may be a DeBakey dual roller, centrifugal pump. Although each of the pumps generate a negative inlet and positive outlet pressure their effect on the blood may be pronounced. Investigators have compared the use of dual roller versus centrifugal pump and have reported lower levels of hemolysis, serum free hemoglobin, comparatively increased platelet count and activity, increased protaglandin and decreased thromboxane levels when engaging the use of a centrifugal pump (Chi). ARTERIAL CANNULA/ VENOUS CATHETER SIZES Aortic or arterial cannula's are usually inserted into the ascending aorta. Alternate routes are the femoral artery, or ductus arteriosus during repair of hypoplastic left heart syndrome (Stammers). The recommended flowrate varies with the manufacturer according to the length and internal diameter of the conduit whic is a function of Poiseuille's Law (Green p.245, Reed and Stafford p. 157). Some cannulas with identical French sizes, but different manufacturer's may vary in flowrates by 40% (Van Meurs). An aortic or arterial cannula is selected which will provide the recommended ECC perfusion flowrate without excessive pressure gradients and resistance approaching Reynold's number, which may lead to increased shear stress with resulting hemolysis. A pressure gradient of < 100 mm. Hg. is preferable(Hope, Molina, Reed and Stafford p.160, Van Meurs). Venous cannula's may be right-angled or straight at the tip. The body may be wire-wound or non-wired. Venous catheters must be able to provide total right heart drainage (Molina). The average height difference between the tip of the venous cannula, situated approximately at the mid- axillary level to the venous drainage port of the oxygenator is approximately 18-20". This differential will produce an average negative hydrostatic pressure of 34-37 mm. Hg. or 46- 51 centimeters of water (CWP) pressure. The hydrostatic pressure differential may be altered by augmenting the height of the column to enhance or reduce venous return (Frazier, Molina). The following table should serve as an approximate guide to arterial cannula and venous catheter selections. It is recommended, however, that the performance of the selected conduits are obtained from the manufacturer. BSA (M2)FLOWRATE (ml./min.)ARTERIAL CANNULA (mm)VENOUS CANNULA (mm)

0.12402.04.0

0.37202.04.0

0.512002.04.0

0.716803.05.0

0.921603.56.0

1.024004.06.0

1.536004.56.0

1.638405.07.0

1.840006.5-8.011.0-15.0

Adopted from K.M. Taylor, Cardiopulmonary Bypass; Principles and Management, Williams and Wilkins, Batimore, MD, p. 117, 1990.

ARTERIAL/ VENOUS CIRCUIT The arterial/venous circuit is designed to permit the maximum calculated blood flowrate without excessive line pressures from resistance, adequate venous drainage without shunting to the right heart while eliminating the need, or minimizing, blood product requirements (Courtney, Molina). Accidental reversal of arterial and venous lines to the arterial cannula and venous catheters with tubing of the same caliber has been reported. The practice of selecting identical arterial/ venous tubing calibers is cautioned or discouraged to deter the possibility of this hazard. The pump boot or "raceway" tubing is the portion of the ECC circuit which comes in contact with the dual roller, or DeBakey, pumphead to provide negative flow at the inlet and positive flow on the outlet ports of the tubing "raceway". Some tubing boots consist of silastic others of polyvinyl chloride. Provided is a general guide to arterial, venous and pump boot tubing based on flowrate requirements. DIAMETER(in.)CALCULATED FLOW(ml./min.)

Arterial Line3/160-1100

1/40-2000

3/80-6500

Venous Line1/40-1100

3/80-2300

1/20-6500

Tubing Pump Boot(Raceway Tubing) 1/40-1100 Infant

3/81000-2300 Pediatric

1/22300-6500 Adult

Heparin bonded or coated circuits are available from several manufacturer's. These circuits present evidence of increased biocompatability with a corresponding decrease in heparin requirement as well a compliment activation (Bennett, Stenach, von Segesser).

BUBBLER vs. MEMBRANE OXYGENATOR The debate continues concerning the use of bubbler versus membrane oxygenators. Many authors have reported that there is virtually no difference in the two systems for procedures lasting less than three hours. However, bubbler oxygenators have a direct blood to gas (100% oxygen) interface which is not conducive to extended cardiopulmonary bypass. The FIO2 is not adjustable in bubbler oxygenators which is a concern during ECC of the premature infant. Retrolental Fibroplasia is causes by hyperoxia induced vasoconstriction of the retinal arteries. Permanent retinal damage has been reported when arterial PO'2 are greater than 110 for longer than 1-2 hours. Semipermeable polypropylene membranes oxygenators, on the other hand, do not have a direct blood to gas interface. These oxygenators may be used exceeding 3 hours of ECC with recommendations of up to 6 hours. The exception is with Extra-corporeal Membrane Oxygenator ECMO applications. The Sci-Med or Kolobow Lung, which is composed of silicone rubber membrane oxygenator is the only alternative for extra- corporeal circulation involving several days. Listed are the more common polypropylene infant/ pediatric membrane oxygenators. Note that the COBE VPCML is a two compartment folded sheet polypropylene membrane. The first compartment may accommodate infant and the second pediatric perfusion flowrates. The two compartments combined may accommodate a large pediatric or small adult ECC flowrates. ECC PRIMING VOLUMES The amount of priming volume for infant and pediatric circuits is dependent on the size of arterial/ venous lines, static prime of the oxygenator and any accessories incorporated such as pre-bypass and arterial filters, hemoconcentrators, ECC arterial line pressure monitoring devices etc. The following are standard priming volumes for average infant and pediatric extracorporeal circuits. INFANT ECC CIRCUIT